The most popular gas discharge tubes in use for displays are the types which use neon gas or a combination of argon and mercury gases. The neon gas when excited glows at a characteristic red color. The combination of argon and mercury gases when excited typically glow in a pale blue color. All other colors used in display signs are typically phosphor-coated tubes in which argon and mercury gases are placed. The argon-mercury vapors are excited which in turn cause the phosphors to glow. The phosphors then glow at the selected color.
Excitation power supplies for gas discharge tubes and in particular for neon or argon-mercury discharge tubes, have been known for many years. The most common form of a discharge supply is a neon light transformer having a 60 Hz, 120 volt AC primary with 60 Hz approximately 10 KVAC secondary which is directly connected to the electrodes attached to either end of the gas discharge tube. A transformer of this size tends to weigh 10-20 pounds due to the massive core, the number of primary and secondary windings and the potting of the transformer in a tar-like material to prevent arcing. This results in a very large, bulky and unsightly excitation supply.
More recently, light-weight switching power supplies have been used to step up the 60 Hz, 120 VAC voltage to a higher frequency for exciting gas discharge tubes. In general, the higher switching frequency allows the use of smaller, more light-weight transformers. The switching frequency may be fixed or may be variable as described in U.S. Pat. No. 4,916,362 entitled "Excitation Supply for Gas Discharge Tubes" issued Apr. 10, 1990 and assigned to the same assignee of the present invention, which is hereby incorporated by reference.
A high frequency excitation supply attached to a gas discharge tube may cause a "bubble effect". This effect varies according to the length and volume of the gas discharge tube, the gas pressure, the temperature and type of gas used in the tube, and other factors. The bubble effect is caused by a standing wave appearing at a high frequency within the discharge tube resulting in alternate areas of light and dark in the tube. The standing wave may not be exactly matched to the length of the tube resulting in a scrolling or crawling bubble effect in which the bubbles slowly move toward one end of the tube. This may be a desirable effect in some gas discharge tube displays but, in general, it is undesirable for display tubes. The problem of the bubble effect is that its appearance is unpredictable because of the number of variables which may cause the bubble effect.
One solution to the bubble effect is to place a DC bias across the tube on top of the high-frequency excitation voltage. The DC bias helps eliminate the bubble effect in most gas discharge tubes, but creates another undesirable effect in argon-mercury gas discharge tubes. A DC bias in an argon-mercury gas discharge tubes causes a slow migration of the mercury to one electrode over time. This disproportionate distribution of mercury results in a dimming of the tube at one end. Hence the DC bias approach for eliminating the bubble effect in argon-mercury tubes may be unacceptable.
There is a need in the prior art, therefore, for a single high frequency switching gas discharge tube supply which may be used for either argon-mercury gas discharge tubes or neon gas discharge tubes. Such a supply should effectively suppress or eliminates the "bubble effect" in these types of gas discharge tubes without causing the migration of mercury toward one electrode over time in an argon-mercury gas discharge tube.